The invention relates to a method of manufacturing polyethylene fibers by high-speed spinning of solutions of ultra-high-molecular-weight polyethylene, thereby producing fibers which are quite suitable for use as industrial yarns, for reinforcing plastics in general, and the like, because of their good strengths and their high modulus.
It is known that fibers and industrial yarns can be made from a number of polymers such as regenerated cellulose, polyester, polyamides, and the like. In all of these methods, the goal is to produce fibers with high strengths, high moduli, especially high initial moduli, and elongation at break which is as small as possible. In addition, the goal is to work at the highest possible production speeds using the simplest procedures possible.
There have been many attempts to produce yarns of this kind from polyethylene which, because of its chemical structure has a number of advantages over polymers like those produced by polycondensation. For example, there is no danger of hydrolysis, which is frequently observed in the ester bonds or amide bonds of polyesters and polyamides. In addition, as a synthetic material that can be manufactured in practically unlimited quantities, polyethylene is less prone to fluctuations in supply and demand, as is the case for cellulose, quite apart from the fact that the supply of raw materials for cellulose is becoming increasingly endangered by the decimation of the forests.
The simplest procedure involves making polyethylene fibers by the melt-spinning process. However, there are limits on melt-spinning polyethylene because, as the molecular weights, which are important for high strength and moduli, increase, the viscosity of the melts increases to the point where they become difficult to spin. The spinning temperature cannot be increased arbitrarily because there is a risk of the polyethylene decomposing at temperatures above approximately 240.degree. C. As molecular weights increase, the elasticity of the polymer melts increases as well, and this can lead to problems, especially at higher extrusion speeds.
Efforts have also been made to overcome these disadvantages by spinning polyethylene solutions into fibers. However, in these methods as well, similar problems arise because the viscosity and elasticity increase considerably with the molecular weight of the dissolved polymer, even in solutions.
In Dutch Disclosure Document 79/04990, a method for manufacturing polyethylene fibers with high strength and high modulus is described, in which process, as is especially clear from the examples, solutions of relatively low concentrations are used. In order to obtain satisfactory mechanical properties, it is necessary to stretch the fibers while hot after spinning, winding, and extracting, thus reducing the productivity of the method.
In "Polymer Bulletin," Volume 16, pages 167-174, 1986, Pennings et al. describe how ultra-high-molecular-weight polyethylene can be spun under various conditions. However, in order for the polyethylene fibers to exhibit usable mechanical properties, the fibers, as in the method described in Dutch Disclosure Document 79/04990, must be stretched, with the fibers also being extracted before stretching.
Although many methods are known for producing polyethylene fibers by spinning ultra-high-molecular-weight polyethylene, there is still a need for improved methods which in particular ensure increased productivity and in which it is not necessary to follow spinning and winding by stretching to obtain usable mechanical properties.